Thursday, May 10, 2012

LROC: Boulder on the Edge

A boulder situated on the edge of the rim of the unnamed nested within Grotrian,
partially covered with a veneer of impact melt. Field of view is roughly 348 by 600 meters, LROC Narrow Angle Camera (NAC) observation M169738975L, orbit 10148, September 4, 2011, angle of incidence 67° at 0.59 meter resolution, from 56.66 kilometers altitude [NASA/GSFC/Arizona State University].

Sarah BradenLROC News System

A boulder rests on the rim of an unnamed fresh crater, whose center is located at 66.615°S, 128.611°E. The boulder is partially covered in a layer of impact melt (dark smooth material). Small pieces of bare rock are visible in the impact melt as high reflectance dots in the relatively lower reflectance melt deposit.

The boulder lies between fractured impact melt (outside the crater rim - left side of the image) and the interior rim of the crater (right side of the image). It appears that pieces of the main boulder broke off and tumbled down the interior crater wall leaving a trail of debris. Some of the impact melt deposit has also crumbled away and fallen towards the interior crater wall.

The sequence of events leading to this geologic scene probably progressed very quickly. First, the impact which formed the crater occurred, generating both the impact melt and the boulders. The boulders and the impact melt were emplaced almost simultaneously. Usually geologists use stratigraphic relationships to determine the order in which events took place, but in this case it is impossible to tell whether the boulder was emplaced first and then the impact melt, or vice versa. In the case where the boulder was emplaced second, it is easy to imagine the impact melt splashing or flowing to partially cover the boulder with a thin veneer of impact melt.

LROC Wide Angle Camera (WAC) context image of Grotrian crater as surveyed last November 2011, viewed at an average 68 meter resolution, and under an illumination incidence angle of 67° from around 50 kilometers altitude. The yellow arrow marks the location of a boulder perched on the rim of the nested unnamed crater superpositioned on the larger Grotrian impact and the subject of the LROC Featured Image released May 10, 2012 [NASA/GSFC/Arizona State University]..

Deep South farside. Simulated ILIADS (LMMP) application oblique perspective from 85 km over the interior of Schrödinger basin northeast to Vallis Planck and the Grotrian crater group, detailed in LROC Featured Images released the week of May 7, 2012. The stark transition from highland to Schrödinger's floor is shown in designated elevations relative to the Moon's global mean average [NASA/GSFC/Arizona State University].